1. Field of the Invention
The field of the invention is that of the transmission of data on a telephone network.
More specifically, the invention relates to a method and a system enabling the generation, by means of voice frequencies, of a sequence of characters and the high-speed transmission of these characters.
In the present description, the term "sequence of characters" is understood to mean a sequence of at least two characters, especially but not exclusively a telephone number, an identification code or enciphering data elements.
The invention has many applications. Thus, the method and system according to the invention may be used to secure long-distance electronic transactions, especially those pertaining to telephone services for which it is essential to be able to speedily identify the provider of the telephone service, It can be applied more particularly when the long-distance electronic transactions are secured by means of portable objects having a credit card format, sending out voice signals and designed to be used in combination with telephone receivers.
2. Description of the Prior Art
Conventionally, the multi-frequency encoding used to transmit dialling, identification and enciphering information on a telephone network is a dual tone multi-frequency or DTMF encoding.
With this DTMF encoding, each character is encoded in the form of a combination of two frequencies (a high frequency and a low frequency) that are transmitted simultaneously, Each of the two frequencies transmitted is chosen from amongst two groups of four frequencies (namely a group of four low frequencies and a group of four high frequencies). One of the frequencies is chosen from one of the groups and the other frequency is chosen from the other group. It is thus possible to encode 4.times.4=16 different characters, i.e. generally the ten digits from 0 to 9 and six additional characters. Owing to the fact that they are located in the telephone passband (from 100 to 3300 Hz), these frequencies are generally called voice or acoustic frequencies. They enable the transmission of the telephone number of the called party and generally, in a conversation stage, of various other data elements (especially identification information). In general, DTMF encoding is implemented by means of a multi-frequency keyboard of a telephone set. The user enters the sequence of characters (for example the digits of a telephone number) by means of his keyboard. Each stroke on this keyboard enables the encoding and dispatch of one character.
There is also a known way of using DTMF encoding in a portable object (for example with a credit card format or in the form of tokens or pocket calculators) of the type designed to be used in combination with a telephone set. In this case, the user applies his portable object against the telephone set, and then activates it (for example by pressing a button designed for this purpose). The portable object then automatically enters a sequence of characters such as for example an identification key designed to be verified by a vocal server which is at the called number.
In general, DTMF encoding enables the transmission of data elements at a speed of 4 to 8 characters per second. While this speed is generally enough for the transmission of dialling information, it is not so for all information elements and especially for the identification codes. Indeed, the number of characters that may be contained in an identification code is sometimes fairly great, Owing to the low speed of transmission, the transmission time is then extremely (and excessively) long for normal use by a private party.
As can be seen with reference to FIGS. 1 to 4 which describe the essential characteristics of DTMF transmission, it is necessary to set up a blank between the transmission of two successive characters. Indeed, when two successive characters are identical, the two DTMF frequencies used to encode this character are the same. Consequently, if a blank is not inserted between two identical successive characters, one of them may be lost at the time of the decoding. The existence of this blank, whose duration is comparable to the period of transmission of a character, reduces the theoretical speed of transmission of the characters by half.
The technical problem to which the present invention proposes to provide a solution is that of the at least partial elimination of the blanks, the goal being to increase the speed of transmission of the characters.